Directory Deletion Without Concern for Order

With traditional file system kernels, to delete a directory a user must first traverse the entire directory and delete files and subdirectories from the bottom up, starting with the leafs (or leaves). Only when all of the contents of the parent directory have been manually deleted can the parent directory be deleted, for example using the UNIX command rmdir. The remove directory command, rmdir directory_name, will only remove an empty directory. There is a UNIX command that will remove a directory and all of the contents of the directory, however, these commands require the user specify the entire directory tree. In addition, the operating system that is interpreting the command must still communicate with the file system to do the directory tree tracing from the top down, and removal of the leafs from bottom-up, which incurs a lot of communication overhead. Other equivalent commands for deleting an entire tree for other operating systems exist, but in all of these alternatives deletion occurs from bottom-up through the tree after the directory tree is first traced from top-down. Each subdirectory is only deleted after it is emptied. Deletion of the top directory must wait until all subdirectories below it have been emptied. The above mechanisms while arguably suitable for hard disk drives, are not optimized for solid-state media.

Solid-state memory, such as flash, is currently in use in solid-state drives (SSD) to augment or replace conventional hard disk drives (HDD), writable CD (compact disk) or writable DVD (digital versatile disk) drives, collectively known as spinning media, and tape drives, for storage of large amounts of data. Flash and other solid-state memories have characteristics that differ from spinning media. Yet, many solid-state drives are designed to conform to hard disk drive standards for compatibility reasons, which makes it difficult to provide enhanced features or take advantage of unique aspects of flash and other solid-state memory. It is within this context that the embodiments arise.

In some embodiments, a method of distributed file deletion, performed by a storage system, is provided. The method includes receiving, at the storage system, a request to delete a directory and contents of the directory and adding the directory to a first set, listed in a memory in the storage system. The method includes operating on the first set, by examining each directory in the first set to identify subdirectories, adding each identified subdirectory to the first set as a directory, and adding each examined directory to a second set listed in the memory. The method includes deleting in a distributed manner across the storage system without concern for order, contents of directories, and the directories, listed in the second set. The method may be embodied on a computer readable medium or executed by a storage system in some embodiments.

Other aspects and advantages of the embodiments will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments.

Various mechanisms described herein efficiently delete a directory and an entire directory tree extending from the specified directory, without requiring the user to manually specify an entire directory tree or have an operating system or file system communicate requests for deletion of each file and each subdirectory to a storage system. Instead of tracing a directory tree from top-down, then deleting leafs from bottom-up and only deleting subdirectories when empty, as is usually the case in file systems, various embodiments of a storage system perform iterative search and destroy processes and background deletions in parallel for greater efficiency and decreased latency in response to a request to delete a directory tree. In some embodiments, a special-named directory is established for deletion of an entire directory tree.show various embodiments of a storage cluster, with storage nodes and solid-state storage units suitable for embodiments that practice distributed directory and file deletion.show aspects of distributed file deletion, and distributed directory deletion.

The embodiments below describe a storage cluster that stores user data, such as user data originating from one or more user or client systems or other sources external to the storage cluster. The storage cluster distributes user data across storage nodes housed within a chassis, using erasure coding and redundant copies of metadata. Erasure coding refers to a method of data protection or reconstruction in which data is stored across a set of different locations, such as disks, storage nodes or geographic locations. Flash memory is one type of solid-state memory that may be integrated with the embodiments, although the embodiments may be extended to other types of solid-state memory or other storage medium, including non-solid state memory. Control of storage locations and workloads are distributed across the storage locations in a clustered peer-to-peer system. Tasks such as mediating communications between the various storage nodes, detecting when a storage node has become unavailable, and balancing I/Os (inputs and outputs) across the various storage nodes, are all handled on a distributed basis. Data is laid out or distributed across multiple storage nodes in data fragments or stripes that support data recovery in some embodiments. Ownership of data can be reassigned within a cluster, independent of input and output patterns. This architecture described in more detail below allows a storage node in the cluster to fail, with the system remaining operational, since the data can be reconstructed from other storage nodes and thus remain available for input and output operations. In various embodiments, a storage node may be referred to as a cluster node, a blade, or a server.

The storage cluster is contained within a chassis, i.e., an enclosure housing one or more storage nodes. A mechanism to provide power to each storage node, such as a power distribution bus, and a communication mechanism, such as a communication bus that enables communication between the storage nodes are included within the chassis. The storage cluster can run as an independent system in one location according to some embodiments. In one embodiment, a chassis contains at least two instances of both the power distribution and the communication bus which may be enabled or disabled independently. The internal communication bus may be an Ethernet bus, however, other technologies such as Peripheral Component Interconnect (PCI) Express, InfiniBand, and others, are equally suitable. The chassis provides a port for an external communication bus for enabling communication between multiple chassis, directly or through a switch, and with client systems. The external communication may use a technology such as Ethernet, InfiniBand, Fibre Channel, etc. In some embodiments, the external communication bus uses different communication bus technologies for inter-chassis and client communication. If a switch is deployed within or between chassis, the switch may act as a translation between multiple protocols or technologies. When multiple chassis are connected to define a storage cluster, the storage cluster may be accessed by a client using either proprietary interfaces or standard interfaces such as network file system (NFS), common internet file system (CIFS), small computer system interface (SCSI) or hypertext transfer protocol (HTTP). Translation from the client protocol may occur at the switch, chassis external communication bus or within each storage node.

Each storage node may be one or more storage servers and each storage server is connected to one or more non-volatile solid state memory units, which may be referred to as storage units or storage devices. One embodiment includes a single storage server in each storage node and between one to eight non-volatile solid state memory units, however this one example is not meant to be limiting. The storage server may include a processor, dynamic random access memory (DRAM) and interfaces for the internal communication bus and power distribution for each of the power buses. Inside the storage node, the interfaces and storage unit share a communication bus, e.g., PCI Express, in some embodiments. The non-volatile solid state memory units may directly access the internal communication bus interface through a storage node communication bus, or request the storage node to access the bus interface. The non-volatile solid state memory unit contains an embedded central processing unit (CPU), solid state storage controller, and a quantity of solid state mass storage, e.g., between 2-32 terabytes (TB) in some embodiments. An embedded volatile storage medium, such as DRAM, and an energy reserve apparatus are included in the non-volatile solid state memory unit. In some embodiments, the energy reserve apparatus is a capacitor, super-capacitor, or battery that enables transferring a subset of DRAM contents to a stable storage medium in the case of power loss. In some embodiments, the non-volatile solid state memory unit is constructed with a storage class memory, such as phase change or magnetoresistive random access memory (MRAM) that substitutes for DRAM and enables a reduced power hold-up apparatus.

One of many features of the storage nodes and non-volatile solid state storage is the ability to proactively rebuild data in a storage cluster. The storage nodes and non-volatile solid state storage can determine when a storage node or non-volatile solid state storage in the storage cluster is unreachable, independent of whether there is an attempt to read data involving that storage node or non-volatile solid state storage. The storage nodes and non-volatile solid state storage then cooperate to recover and rebuild the data in at least partially new locations. This constitutes a proactive rebuild, in that the system rebuilds data without waiting until the data is needed for a read access initiated from a client system employing the storage cluster. These and further details of the storage memory and operation thereof are discussed below.

is a perspective view of a storage cluster, with multiple storage nodesand internal solid-state memory coupled to each storage node to provide network attached storage or storage area network, in accordance with some embodiments. A network attached storage, storage area network, or a storage cluster, or other storage memory, could include one or more storage clusters, each having one or more storage nodes, in a flexible and reconfigurable arrangement of both the physical components and the amount of storage memory provided thereby. The storage clusteris designed to fit in a rack, and one or more racks can be set up and populated as desired for the storage memory. The storage clusterhas a chassishaving multiple slots. It should be appreciated that chassismay be referred to as a housing, enclosure, or rack unit. In one embodiment, the chassishas fourteen slots, although other numbers of slots are readily devised. For example, some embodiments have four slots, eight slots, sixteen slots, thirty-two slots, or other suitable number of slots. Each slotcan accommodate one storage nodein some embodiments. Chassisincludes flapsthat can be utilized to mount the chassison a rack. Fansprovide air circulation for cooling of the storage nodesand components thereof, although other cooling components could be used, or an embodiment could be devised without cooling components. A switch fabriccouples storage nodeswithin chassistogether and to a network for communication to the memory. In an embodiment depicted in, the slotsto the left of the switch fabricand fansare shown occupied by storage nodes, while the slotsto the right of the switch fabricand fansare empty and available for insertion of storage nodefor illustrative purposes. This configuration is one example, and one or more storage nodescould occupy the slotsin various further arrangements. The storage node arrangements need not be sequential or adjacent in some embodiments. Storage nodesare hot pluggable, meaning that a storage nodecan be inserted into a slotin the chassis, or removed from a slot, without stopping or powering down the system. Upon insertion or removal of storage nodefrom slot, the system automatically reconfigures in order to recognize and adapt to the change. Reconfiguration, in some embodiments, includes restoring redundancy and/or rebalancing data or load.

Each storage nodecan have multiple components. In the embodiment shown here, the storage nodeincludes a printed circuit boardpopulated by a CPU, i.e., processor, a memorycoupled to the CPU, and a non-volatile solid state storagecoupled to the CPU, although other mountings and/or components could be used in further embodiments. The memoryhas instructions which are executed by the CPUand/or data operated on by the CPU. As further explained below, the non-volatile solid state storageincludes flash or, in further embodiments, other types of solid-state memory.

Referring to, storage clusteris scalable, meaning that storage capacity with non-uniform storage sizes is readily added, as described above. One or more storage nodescan be plugged into or removed from each chassis and the storage cluster self-configures in some embodiments. Plug-in storage nodes, whether installed in a chassis as delivered or later added, can have different sizes. For example, in one embodiment a storage nodecan have any multiple of 4 TB, e.g., 8 TB, 12 TB, 16 TB, 32 TB, etc. In further embodiments, a storage nodecould have any multiple of other storage amounts or capacities. Storage capacity of each storage nodeis broadcast, and influences decisions of how to stripe the data. For maximum storage efficiency, an embodiment can self-configure as wide as possible in the stripe, subject to a predetermined requirement of continued operation with loss of up to one, or up to two, non-volatile solid state storage unitsor storage nodeswithin the chassis.

is a block diagram showing a communications interconnectand power distribution buscoupling multiple storage nodes. Referring back to, the communications interconnectcan be included in or implemented with the switch fabricin some embodiments. Where multiple storage clustersoccupy a rack, the communications interconnectcan be included in or implemented with a top of rack switch, in some embodiments. As illustrated in, storage clusteris enclosed within a single chassis. External portis coupled to storage nodesthrough communications interconnect, while external portis coupled directly to a storage node. External power portis coupled to power distribution bus. Storage nodesmay include varying amounts and differing capacities of non-volatile solid state storageas described with reference to. In addition, one or more storage nodesmay be a compute only storage node as illustrated in. Authoritiesare implemented on the non-volatile solid state storages, for example as lists or other data structures stored in memory. In some embodiments the authorities are stored within the non-volatile solid state storageand supported by software executing on a controller or other processor of the non-volatile solid state storage. In a further embodiment, authoritiesare implemented on the storage nodes, for example as lists or other data structures stored in the memoryand supported by software executing on the CPUof the storage node. Authoritiescontrol how and where data is stored in the non-volatile solid state storagesin some embodiments. This control assists in determining which type of erasure coding scheme is applied to the data, and which storage nodeshave which portions of the data. Each authoritymay be assigned to a non-volatile solid state storage. Each authority may control a range of inode numbers, segment numbers, or other data identifiers which are assigned to data by a file system, by the storage nodes, or by the non-volatile solid state storage, in various embodiments.

Every piece of data, and every piece of metadata, has redundancy in the system in some embodiments. In addition, every piece of data and every piece of metadata has an owner, which may be referred to as an authority. If that authority is unreachable, for example through failure of a storage node, there is a plan of succession for how to find that data or that metadata. In various embodiments, there are redundant copies of authorities. Authoritieshave a relationship to storage nodesand non-volatile solid state storagein some embodiments. Each authority, covering a range of data segment numbers or other identifiers of the data, may be assigned to a specific non-volatile solid state storage. In some embodiments the authoritiesfor all of such ranges are distributed over the non-volatile solid state storagesof a storage cluster. Each storage nodehas a network port that provides access to the non-volatile solid state storage(s)of that storage node. Data can be stored in a segment, which is associated with a segment number and that segment number is an indirection for a configuration of a RAID (redundant array of independent disks) stripe in some embodiments. The assignment and use of the authoritiesthus establishes an indirection to data. Indirection may be referred to as the ability to reference data indirectly, in this case via an authority, in accordance with some embodiments. A segment identifies a set of non-volatile solid state storageand a local identifier into the set of non-volatile solid state storagethat may contain data. In some embodiments, the local identifier is an offset into the device and may be reused sequentially by multiple segments. In other embodiments the local identifier is unique for a specific segment and never reused. The offsets in the non-volatile solid state storageare applied to locating data for writing to or reading from the non-volatile solid state storage(in the form of a RAID stripe). Data is striped across multiple units of non-volatile solid state storage, which may include or be different from the non-volatile solid state storagehaving the authorityfor a particular data segment.

If there is a change in where a particular segment of data is located, e.g., during a data move or a data reconstruction, the authorityfor that data segment should be consulted, at that non-volatile solid state storageor storage nodehaving that authority. In order to locate a particular piece of data, embodiments calculate a hash value for a data segment or apply an inode number or a data segment number. The output of this operation points to a non-volatile solid state storagehaving the authorityfor that particular piece of data. In some embodiments there are two stages to this operation. The first stage maps an entity identifier (ID), e.g., a segment number, inode number, or directory number to an authority identifier. This mapping may include a calculation such as a hash or a bit mask. The second stage is mapping the authority identifier to a particular non-volatile solid state storage, which may be done through an explicit mapping. The operation is repeatable, so that when the calculation is performed, the result of the calculation repeatably and reliably points to a particular non-volatile solid state storagehaving that authority. The operation may include the set of reachable storage nodes as input. If the set of reachable non-volatile solid state storage units changes the optimal set changes. In some embodiments, the persisted value is the current assignment (which is always true) and the calculated value is the target assignment the cluster will attempt to reconfigure towards. This calculation may be used to determine the optimal non-volatile solid state storagefor an authority in the presence of a set of non-volatile solid state storagethat are reachable and constitute the same cluster. The calculation also determines an ordered set of peer non-volatile solid state storagethat will also record the authority to non-volatile solid state storage mapping so that the authority may be determined even if the assigned non-volatile solid state storage is unreachable. A duplicate or substitute authoritymay be consulted if a specific authorityis unavailable in some embodiments.

With reference to, two of the many tasks of the CPUon a storage nodeare to break up write data, and reassemble read data. When the system has determined that data is to be written, the authorityfor that data is located as above. When the segment ID for data is already determined the request to write is forwarded to the non-volatile solid state storagecurrently determined to be the host of the authoritydetermined from the segment. The host CPUof the storage node, on which the non-volatile solid state storageand corresponding authorityreside, then breaks up or shards the data and transmits the data out to various non-volatile solid state storage. The transmitted data is written as a data stripe in accordance with an erasure coding scheme. In some embodiments, data is requested to be pulled, and in other embodiments, data is pushed. In reverse, when data is read, the authorityfor the segment ID containing the data is located as described above. The host CPUof the storage nodeon which the non-volatile solid state storageand corresponding authorityreside requests the data from the non-volatile solid state storage and corresponding storage nodes pointed to by the authority. In some embodiments the data is read from flash storage as a data stripe. The host CPUof storage nodethen reassembles the read data, correcting any errors (if present) according to the appropriate erasure coding scheme, and forwards the reassembled data to the network. In further embodiments, some or all of these tasks can be handled in the non-volatile solid state storage. In some embodiments, the segment host requests the data be sent to storage nodeby requesting pages from storage and then sending the data to the storage node making the original request.

In some systems, for example in UNIX-style file systems, data is handled with an index node or inode, which specifies a data structure that represents an object in a file system. The object could be a file or a directory, for example. Metadata may accompany the object, as attributes such as permission data and a creation timestamp, among other attributes. A segment number could be assigned to all or a portion of such an object in a file system. In other systems, data segments are handled with a segment number assigned elsewhere. For purposes of discussion, the unit of distribution is an entity, and an entity can be a file, a directory or a segment. That is, entities are units of data or metadata stored by a storage system. Entities are grouped into sets called authorities. Each authority has an authority owner, which is a storage node that has the exclusive right to update the entities in the authority. In other words, a storage node contains the authority, and that the authority, in turn, contains entities.

A segment is a logical container of data in accordance with some embodiments. A segment is an address space between medium address space and physical flash locations, i.e., the data segment number, are in this address space. Segments may also contain meta-data, which enable data redundancy to be restored (rewritten to different flash locations or devices) without the involvement of higher level software. In one embodiment, an internal format of a segment contains client data and medium mappings to determine the position of that data. Each data segment is protected, e.g., from memory and other failures, by breaking the segment into a number of data and parity shards, where applicable. The data and parity shards are distributed, i.e., striped, across non-volatile solid state storagecoupled to the host CPUs(See) in accordance with an erasure coding scheme. Usage of the term segments refers to the container and its place in the address space of segments in some embodiments. Usage of the term stripe refers to the same set of shards as a segment and includes how the shards are distributed along with redundancy or parity information in accordance with some embodiments.

A series of address-space transformations takes place across an entire storage system. At the top are the directory entries (file names) which link to an inode. Inodes point into medium address space, where data is logically stored. Medium addresses may be mapped through a series of indirect mediums to spread the load of large files, or implement data services like deduplication or snapshots. Medium addresses may be mapped through a series of indirect mediums to spread the load of large files, or implement data services like deduplication or snapshots. Segment addresses are then translated into physical flash locations. Physical flash locations have an address range bounded by the amount of flash in the system in accordance with some embodiments. Medium addresses and segment addresses are logical containers, and in some embodiments use a 128 bit or larger identifier so as to be practically infinite, with a likelihood of reuse calculated as longer than the expected life of the system. Addresses from logical containers are allocated in a hierarchical fashion in some embodiments. Initially, each non-volatile solid state storage unitmay be assigned a range of address space. Within this assigned range, the non-volatile solid state storageis able to allocate addresses without synchronization with other non-volatile solid state storage.

Data and metadata is stored by a set of underlying storage layouts that are optimized for varying workload patterns and storage devices. These layouts incorporate multiple redundancy schemes, compression formats and index algorithms. Some of these layouts store information about authorities and authority masters, while others store file metadata and file data. The redundancy schemes include error correction codes that tolerate corrupted bits within a single storage device (such as a NAND flash chip), erasure codes that tolerate the failure of multiple storage nodes, and replication schemes that tolerate data center or regional failures. In some embodiments, low density parity check (LDPC) code is used within a single storage unit. Reed-Solomon encoding is used within a storage cluster, and mirroring is used within a storage grid in some embodiments. Metadata may be stored using an ordered log structured index (such as a Log Structured Merge Tree), and large data may not be stored in a log structured layout.

In order to maintain consistency across multiple copies of an entity, the storage nodes agree implicitly on two things through calculations: (1) the authority that contains the entity, and (2) the storage node that contains the authority. The assignment of entities to authorities can be done by pseudo randomly assigning entities to authorities, by splitting entities into ranges based upon an externally produced key, or by placing a single entity into each authority. Examples of pseudorandom schemes are linear hashing and the Replication Under Scalable Hashing (RUSH) family of hashes, including Controlled Replication Under Scalable Hashing (CRUSH). In some embodiments, pseudo-random assignment is utilized only for assigning authorities to nodes because the set of nodes can change. The set of authorities cannot change so any subjective function may be applied in these embodiments. Some placement schemes automatically place authorities on storage nodes, while other placement schemes rely on an explicit mapping of authorities to storage nodes. In some embodiments, a pseudorandom scheme is utilized to map from each authority to a set of candidate authority owners. A pseudorandom data distribution function related to CRUSH may assign authorities to storage nodes and create a list of where the authorities are assigned. Each storage node has a copy of the pseudorandom data distribution function, and can arrive at the same calculation for distributing, and later finding or locating an authority. Each of the pseudorandom schemes requires the reachable set of storage nodes as input in some embodiments in order to conclude the same target nodes. Once an entity has been placed in an authority, the entity may be stored on physical devices so that no expected failure will lead to unexpected data loss. In some embodiments, rebalancing algorithms attempt to store the copies of all entities within an authority in the same layout and on the same set of machines.

Examples of expected failures include device failures, stolen machines, datacenter fires, and regional disasters, such as nuclear or geological events. Different failures lead to different levels of acceptable data loss. In some embodiments, a stolen storage node impacts neither the security nor the reliability of the system, while depending on system configuration, a regional event could lead to no loss of data, a few seconds or minutes of lost updates, or even complete data loss.

In the embodiments, the placement of data for storage redundancy is independent of the placement of authorities for data consistency. In some embodiments, storage nodes that contain authorities do not contain any persistent storage. Instead, the storage nodes are connected to non-volatile solid state storage units that do not contain authorities. The communications interconnect between storage nodes and non-volatile solid state storage units consists of multiple communication technologies and has non-uniform performance and fault tolerance characteristics. In some embodiments, as mentioned above, non-volatile solid state storage units are connected to storage nodes via PCI express, storage nodes are connected together within a single chassis using Ethernet backplane, and chassis are connected together to form a storage cluster. Storage clusters are connected to clients using Ethernet or fiber channel in some embodiments. If multiple storage clusters are configured into a storage grid, the multiple storage clusters are connected using the Internet or other long-distance networking links, such as a “metro scale” link or private link that does not traverse the internet.

Authority owners have the exclusive right to modify entities, to migrate entities from one non-volatile solid state storage unit to another non-volatile solid state storage unit, and to add and remove copies of entities. This allows for maintaining the redundancy of the underlying data. When an authority owner fails, is going to be decommissioned, or is overloaded, the authority is transferred to a new storage node. Transient failures make it non-trivial to ensure that all non-faulty machines agree upon the new authority location. The ambiguity that arises due to transient failures can be achieved automatically by a consensus protocol such as Paxos, hot-warm failover schemes, via manual intervention by a remote system administrator, or by a local hardware administrator (such as by physically removing the failed machine from the cluster, or pressing a button on the failed machine). In some embodiments, a consensus protocol is used, and failover is automatic. If too many failures or replication events occur in too short a time period, the system goes into a self-preservation mode and halts replication and data movement activities until an administrator intervenes in accordance with some embodiments.

As authorities are transferred between storage nodes and authority owners update entities in their authorities, the system transfers messages between the storage nodes and non-volatile solid state storage units. With regard to persistent messages, messages that have different purposes are of different types. Depending on the type of the message, the system maintains different ordering and durability guarantees. As the persistent messages are being processed, the messages are temporarily stored in multiple durable and non-durable storage hardware technologies. In some embodiments, messages are stored in RAM, NVRAM and on NAND flash devices, and a variety of protocols are used in order to make efficient use of each storage medium. Latency-sensitive client requests may be persisted in replicated NVRAM, and then later NAND, while background rebalancing operations are persisted directly to NAND.

Persistent messages are persistently stored prior to being transmitted. This allows the system to continue to serve client requests despite failures and component replacement. Although many hardware components contain unique identifiers that are visible to system administrators, manufacturer, hardware supply chain and ongoing monitoring quality control infrastructure, applications running on top of the infrastructure address virtualize addresses. These virtualized addresses do not change over the lifetime of the storage system, regardless of component failures and replacements. This allows each component of the storage system to be replaced over time without reconfiguration or disruptions of client request processing.

In some embodiments, the virtualized addresses are stored with sufficient redundancy. A continuous monitoring system correlates hardware and software status and the hardware identifiers. This allows detection and prediction of failures due to faulty components and manufacturing details. The monitoring system also enables the proactive transfer of authorities and entities away from impacted devices before failure occurs by removing the component from the critical path in some embodiments.

is a multiple level block diagram, showing contents of a storage nodeand contents of a non-volatile solid state storageof the storage node. Data is communicated to and from the storage nodeby a network interface controller (NIC)in some embodiments. Each storage nodehas a CPU, and one or more non-volatile solid state storage, as discussed above. Moving down one level in, each non-volatile solid state storagehas a relatively fast non-volatile solid state memory, such as nonvolatile random access memory (NVRAM), and flash memory. In some embodiments, NVRAMmay be a component that does not require program/erase cycles (DRAM, MRAM, PCM), and can be a memory that can support being written vastly more often than the memory is read from. Moving down another level in, the NVRAMis implemented in one embodiment as high speed volatile memory, such as dynamic random access memory (DRAM), backed up by energy reserve. Energy reserveprovides sufficient electrical power to keep the DRAMpowered long enough for contents to be transferred to the flash memoryin the event of power failure. In some embodiments, energy reserveis a capacitor, super-capacitor, battery, or other device, that supplies a suitable supply of energy sufficient to enable the transfer of the contents of DRAMto a stable storage medium in the case of power loss. The flash memoryis implemented as multiple flash dies, which may be referred to as packages of flash diesor an array of flash dies. It should be appreciated that the flash diescould be packaged in any number of ways, with a single die per package, multiple dies per package (i.e. multichip packages), in hybrid packages, as bare dies on a printed circuit board or other substrate, as encapsulated dies, etc. In the embodiment shown, the non-volatile solid state storagehas a controlleror other processor, and an input output (I/O) portcoupled to the controller. I/O portis coupled to the CPUand/or the network interface controllerof the flash storage node. Flash input output (I/O) portis coupled to the flash dies, and a direct memory access unit (DMA)is coupled to the controller, the DRAMand the flash dies. In the embodiment shown, the I/O port, controller, DMA unitand flash I/O portare implemented on a programmable logic device (PLD), e.g., a field programmable gate array (FPGA). In this embodiment, each flash diehas pages, organized as sixteen kB (kilobyte) pages, and a registerthrough which data can be written to or read from the flash die. In further embodiments, other types of solid-state memory are used in place of, or in addition to flash memory illustrated within flash die.

Storage clusters, in various embodiments as disclosed herein, can be contrasted with storage arrays in general. The storage nodesare part of a collection that creates the storage cluster. Each storage nodeowns a slice of data and computing required to provide the data. Multiple storage nodescooperate to store and retrieve the data. Storage memory or storage devices, as used in storage arrays in general, are less involved with processing and manipulating the data. Storage memory or storage devices in a storage array receive commands to read, write, or erase data. The storage memory or storage devices in a storage array are not aware of a larger system in which they are embedded, or what the data means. Storage memory or storage devices in storage arrays can include various types of storage memory, such as RAM, solid state drives, hard disk drives, etc. The storage unitsdescribed herein have multiple interfaces active simultaneously and serving multiple purposes. In some embodiments, some of the functionality of a storage nodeis shifted into a storage unit, transforming the storage unitinto a combination of storage unitand storage node. Placing computing (relative to storage data) into the storage unitplaces this computing closer to the data itself. The various system embodiments have a hierarchy of storage node layers with different capabilities. By contrast, in a storage array, a controller owns and knows everything about all of the data that the controller manages in a shelf or storage devices. In a storage cluster, as described herein, multiple controllers in multiple storage unitsand/or storage nodescooperate in various ways (e.g., for erasure coding, data sharding, metadata communication and redundancy, storage capacity expansion or contraction, data recovery, and so on).

shows a storage server environment, which uses embodiments of the storage nodesand storage unitsof. In this version, each storage unithas a processor such as controller(see), an FPGA (field programmable gate array), flash memory, and NVRAM(which is super-capacitor backed DRAM, see) on a PCIe (peripheral component interconnect express) board in a chassis(see). The storage unitmay be implemented as a single board containing storage, and may be the largest tolerable failure domain inside the chassis. In some embodiments, up to two storage unitsmay fail and the device will continue with no data loss.

The physical storage is divided into named regions based on application usage in some embodiments. The NVRAMis a contiguous block of reserved memory in the storage unitDRAM, and is backed by NAND flash. NVRAMis logically divided into multiple memory regions written for two as spool (e.g., spool_region). Space within the NVRAMspools is managed by each authorityindependently. Each device provides an amount of storage space to each authority. That authorityfurther manages lifetimes and allocations within that space. Examples of a spool include distributed transactions or notions. When the primary power to a storage unitfails, onboard super-capacitors provide a short duration of power hold up. During this holdup interval, the contents of the NVRAMare flushed to flash memory. On the next power-on, the contents of the NVRAMare recovered from the flash memory.

As for the storage unit controller, the responsibility of the logical “controller” is distributed across each of the blades containing authorities. This distribution of logical control is shown inas a host controller, mid-tier controllerand storage unit controller(s). Management of the control plane and the storage plane are treated independently, although parts may be physically co-located on the same blade. Each authorityeffectively serves as an independent controller. Each authorityprovides its own data and metadata structures, its own background workers, and maintains its own lifecycle.

is a bladehardware block diagram, showing a control plane, compute and storage planes,, and authoritiesinteracting with underlying physical resources, using embodiments of the storage nodesand storage unitsofin the storage server environment of. The control planeis partitioned into a number of authoritieswhich can use the compute resources in the compute planeto run on any of the blades. The storage planeis partitioned into a set of devices, each of which provides access to flashand NVRAMresources.

In the compute and storage planes,of, the authoritiesinteract with the underlying physical resources (i.e., devices). From the point of view of an authority, its resources are striped over all of the physical devices. From the point of view of a device, it provides resources to all authorities, irrespective of where the authorities happen to run. Each authorityhas allocated or has been allocated one or more partitionsof storage memory in the storage units, e.g. partitionsin flash memoryand NVRAM. Each authorityuses those allocated partitionsthat belong to it, for writing or reading user data. Authorities can be associated with differing amounts of physical storage of the system. For example, one authoritycould have a larger number of partitionsor larger sized partitionsin one or more storage unitsthan one or more other authorities. Authoritiesofand authoritiesofrefer to the same construct.

depicts elasticity software layers in bladesof a storage cluster, in accordance with some embodiments. In the elasticity structure, elasticity software is symmetric, i.e., each blade'scompute moduleruns the three identical layers of processes depicted in. Storage managersexecute read and write requests from other bladesfor data and metadata stored in local storage unitNVRAMand flash. Authoritiesfulfill client requests by issuing the necessary reads and writes to the bladeson whose storage unitsthe corresponding data or metadata resides. Endpointsparse client connection requests received from switch fabricsupervisory software, relay the client connection requests to the authoritiesresponsible for fulfillment, and relay the authorities'responses to clients. The symmetric three-layer structure enables the storage system's high degree of concurrency. Elasticity scales out efficiently and reliably in these embodiments. In addition, elasticity implements a unique scale-out technique that balances work evenly across all resources regardless of client access pattern, and maximizes concurrency by eliminating much of the need for inter-blade coordination that typically occurs with conventional distributed locking.

Still referring to, authoritiesrunning in the compute modulesof a bladeperform the internal operations required to fulfill client requests. One feature of elasticity is that authoritiesare stateless, i.e., they cache active data and metadata in their own blades'DRAMs for fast access, but the authorities store every update in their NVRAMpartitions on three separate bladesuntil the update has been written to flash. All the storage system writes to NVRAMare in triplicate to partitions on three separate bladesin some embodiments. With triple-mirrored NVRAMand persistent storage protected by parity and Reed-Solomon RAID checksums, the storage system can survive concurrent failure of two bladeswith no loss of data, metadata, or access to either.

Because authoritiesare stateless, they can migrate between blades. Each authorityhas a unique identifier. NVRAMand flashpartitions are associated with authorities'identifiers, not with the bladeson which they are running in some embodiments. Thus, when an authoritymigrates, the authoritycontinues to manage the same storage partitions from its new location. When a new bladeis installed in an embodiment of the storage cluster, the system automatically rebalances load by:

From their new locations, migrated authoritiespersist the contents of their NVRAMpartitions on flash, process read and write requests from other authorities, and fulfill the client requests that endpointsdirect to them. Similarly, if a bladefails or is removed, the system redistributes its authoritiesamong the system's remaining blades. The redistributed authoritiescontinue to perform their original functions from their new locations.

depicts authoritiesand storage resources in bladesof a storage cluster, in accordance with some embodiments. Each authorityis exclusively responsible for a partition of the flashand NVRAMon each blade. The authoritymanages the content and integrity of its partitions independently of other authorities. Authoritiescompress incoming data and preserve it temporarily in their NVRAMpartitions, and then consolidate, RAID-protect, and persist the data in segments of the storage in their flashpartitions. As the authoritieswrite data to flash, storage managersperform the necessary flash translation to optimize write performance and maximize media longevity. In the background, authorities“garbage collect,” or reclaim space occupied by data that clients have made obsolete by overwriting the data. It should be appreciated that since authorities'partitions are disjoint, there is no need for distributed locking to execute client and writes or to perform background functions.

is an action diagram showing a directory tree and a special-named directoryfor deleting a specified directoryand the entire tree below that directory. In this example, the special-named directoryis “fast_remove”, but the directory could also be named “tree_destroy”, “directory_tree_delete” or any other reserved name to invoke the directory tree deleting properties of the special-named directory. Unlike ordinary directories, the special-named directoryconceals contents from the user, and does not show subdirectories, files, a tree or a partial tree, etc. Moving a directoryto (i.e., under) the special-named directory, for example using a UNIX command mv directory_name special-named_directory, directs or requests the storage system to delete the specified directoryand the entire tree below that directory, i.e., the parent directoryand all subdirectoriesand all filesbelow that directory. Although this is one specific mechanism for requesting a tree deletion, other commands for deleting a tree, with or without specifying a special-named directory, such as “delete_tree directory_name” could be devised as an extension to an operating system or file system.

When the storage system is directed to delete an entire tree, the storage system establishes a trash list, which is then populated with names (or other identifying or addressing information) of directories, subdirectoriesand files to be deleted. In, this process has started, and the specified directory, “dir 1”, and subdirectories“a”, “b” under the specified directoryhave been added to the trash list.

continues the action diagram of, and shows top-down iterative or recursive listing of a directory, subdirectoriesand filesin a trash list, in some embodiments performed by processorson behalf of authorities, in iterative search and destroyprocesses with batchcommunication and background deletions. A multitasking computational system can parcel out threads and processes to perform the iterative search and destroyand the background deletionsin parallel. In various embodiments as described with reference to, authoritiesperform the iterative search and destroyand background deletions. The authoritiescommunicate among themselves, among storage nodesand/or solid state storage unitsin the storage cluster.

In a specific scenario for one embodiment, as shown in, an authorityfor the special-named directorydetermines which authorityis the owner of the inode for the specified directorythat has been moved to the special-named directoryfor tree deletion, and listed in the trash list. The special-named directory owningauthoritycommunicates with the specified directoryowning authority, e.g., by sending a message, to initiate the top-down iterative search and destroyprocesses. Alternatively, authoritiespoll or consult the trash listto determine if a task is available. The specified directoryowning authoritydetermines the contents and respective authoritiesfor the contents of the specified directory, “dir 1”, places those contents (e.g., names or other identifying information or addresses of subdirectoriesor files) on to the trash listand deletes the specified directoryfrom memory and deletes the name of the specified directory, or other identification or addressing information, from the trash list. The link from the specified directoryto whichever directory is above, in this case the root directory “/”, is severed, so that the specified directoryis no longer visible to the file system or to a user. Also, the special-named directoryconceals visibility of the specified directoryor any contents thereof.

Proceeding top-down, the specified directoryowning authoritycommunicates to the authoritiesidentified as owning contents of the specified directory, e.g., by sending messages. Alternatively, authoritiesdiscover what is on the trash list. Those authoritiesproceed in the same iterative top-down manner, identifying contents of their own directories (now subdirectories) and respective authoritiesfor those contents (e.g., filesand/or further subdirectories), placing names, identifying information or addressing of those contents onto the trash listand deleting the directories (e.g., subdirectories) owned by those authoritiesand deleting the directories from the trash list. So, for example, after being contacted by the authorityfor the specified directory“dir 1”, the authorityfor the subdirectory“a” identifies files“A”, “B” and authoritiesfor those files, and communicates to those authorities, while deleting the directory “a” and deleting the name “a” or other identifying or address information for the subdirectoryowned by the authorityfrom the trash list. Similarly, the authorityfor the subdirectory“b” identifies files“A”, “B”, “C” and authoritiesfor those files, and communicates to those authorities, while deleting the directory “b” from memory and deleting the name “b” or other identifying or address information for the subdirectoryowned by that authorityfrom the trash list. Those authoritiesfor the inodes of the filesplace identifying information or addresses for portions of the files, e.g., segment ID numbers or ranges of segment ID numbers, onto the trash list. Respective owners, i.e., authoritiesin some embodiments, of entries on the trash listperform background deletionsof the directory, subdirectoriesand filesor portions of files, deleting these entries from the trash list as the background deletionsare performed. In some embodiments, the requests for performing the iterative search and destroyprocesses, and the background deletions, are communicated among authoritiesin batches.

In, the trash listis shown in various stages, with partial information present and deletions from the trash listoccurring as other entries are added. For example, in one iteration (at top right of), the trash listshows entries for the specified directory“dir 1” and subdirectories“a”, “b” and the file“E” being deleted through batches. Another iteration (at the middle right of) has the trash listshowing the files“A”, “B” from the subdirectory“a” and the files“A”, “B”, “C” from the subdirectory“b”, which will soon be deleted in batches.

With ongoing reference to, one mechanism for employing the trash listis as a communication center for the iterative search and destroy processesand background deletions. When an entry, such as a directory, subdirectory, or file, is placed on the trash list, a background process (e.g., an authorityin some embodiments) can pick up that entry and enumerate the entry, that is, list what are the contents of the entry. In the case of a directoryor subdirectory, the background process lists the contents of the directoryor subdirectory(e.g., more subdirectoriesand/or files), and places corresponding entries onto the trash list. In the case of a file, the background process lists portions of the file, such as are owned by further authoritiesin various embodiments. Once the enumeration has been done for the immediate contents of the directoryor subdirectory(i.e., not the entire depth of the tree), the background process can delete the entry on the trash list and actually delete or commit to deleting the corresponding item, such as a directory, subdirectory, or file. Again in the case of a file, that background process could then either delete the file or contact further authoritieseach of which could delete portions of the file owned by those authorities, or list the portions of the file on the trash listfor deletion by authoritiesthat own those portions of the file, in various embodiments. In some embodiments, the background deletionsare coordinated with garbage collection and recovery of physical memory, for example solid state storage memory. In some embodiments, authoritiesother than owners of inodes could take on some of the iterative search and destroyor background deletions, for example authoritiesthat are not busy performing reads or writes of user data.

A further benefit of the parallelism in the above mechanisms is that authoritiesexchange batches, obtaining tasks according to the trash list, finding entries in directoriesor subdirectorythat other authorities should free, and returning memory space for storage of metadata and user data. In some embodiments, since the freeing up of storage space is happening in parallel, commitment to data writes can also be made in parallel. The authoritiescollectively can determine the amount of memory space made available, and schedule writes in accordance with the memory space available.

It should be appreciated that the deletion of the parent or specified directory“dir 1” referenced in the tree deletion request does not wait for deletion of the leafs at the bottom of the tree (e.g., the filesat the bottom of), and can proceed as soon as the specified directoryis placed on the trash list. Similarly, deletion of subdirectoriescan proceed as soon as these subdirectoriesare placed on the trash list. Or, deletion can occur later. Because the deletion of a directoryor subdirectorydoes not depend on emptying that directoryor subdirectory, the background deletionscan proceed in parallel and in any order. Processing of any subtree can be performed without concern of consistency with other processing of other subtrees. Further examples with taller or wider directory trees, and other names or conventions for directories or files are readily devised in keeping with the teachings herein. The above-described examples and mechanisms can be extended to trees with hundreds, thousands or millions, etc., of subdirectories and files.

With reference back to, some embodiments of the storage clusterhave power loss recovery for the distributed file and directory deletion mechanisms described above. In the case of power loss, the trash listis flushed from NVRAMto flash memory(e.g., along with other data and metadata in the system). Upon restart, the trash list is recovered from the flash memory, back to NVRAM. Then, the iterative search and destroyand background deletionscan continue.

is a flow diagram of a method for distributed directory and file deletion of a directory tree, which can be practiced in the storage cluster of, and in further storage systems, in accordance with some embodiments as described with reference to. Some or all of the actions in the method can be performed by various processors, such as processors in storage nodes or processors in storage units. In an action, a request is received to move a directory under a special-named directory for tree deletion. That is, a request is received to delete a directory and contents. In some embodiments, the request is specific to the special-named directory and a specified directory for tree deletion, and in other embodiments, the request does not mention a special-named directory. In an action, proceeding top-down, directory and contents of the directory are listed to a trash list. In the decision action, it is determined whether there are any subdirectories below the listed directory. If yes, there is at least one subdirectory in the directory, flow proceeds to iterate, and returns to the actionto proceed from each subdirectory in a top-down manner to list the directory and contents of the directory to the trash list. Multiple processes can be spawned in parallel, in some embodiments. If no, there is no subdirectory in the directory, then the process is done listing the entire tree of the directory in the trash list. The actions,,thus show an iterative, recursive, top-down process of listing directories and contents for an entire directory tree to the trash list, for deletion.